The Federal Communications Commission (FCC) has allotted a spectrum of bandwidth in the 60 GHz frequency range (57 to 64 GHz). The Wireless Gigabit Alliance (WiGig) is targeting the standardization of this frequency band which will support data transmission rates up to 7 Gbps. Integrated circuits, formed in semiconductor die, offer high frequency operation in this millimeter wavelength range of frequencies. Some of these integrated circuits utilize Complementary Metal Oxide Semiconductor (CMOS), Silicon-Germanium (SiGe) or Gallium Arsenide (GaAs) technology to form the dice in these designs. Since WiGig transceivers use Digital to Analog Converters (DAC), the reduced power supply impacts the performance of the DAC's.
Complementary Metal Oxide Semiconductor (CMOS) is the primary technology used to construct integrated circuits. N-channel transistors and P-channel transistors (MOS transistor) are used in this technology which uses fine line technology to consistently reduce the channel length of the MOS transistors. Some of the current values for this technology include the channel length being 40 nm, the power supply of VDD equaling 1.2 V and the number of layers of metal levels being 8 or more. This technology typically scales with technology.
CMOS technology delivers a designer the ability to form a very large system level design on one die which is known as a System On a Chip (SOC). The SOC is a complex system with millions, if not billions, of transistors which contain analog circuits and digital circuits. The analog circuits operate purely analog, the digital circuits operate purely digital and these two circuits types can be combined together to form circuits operating in a mixed-signal mode.
For example, digital circuits in their basic form only use digital logic and some examples can be a component comprising at least one; processor, memory, control logic, digital I/O circuit, reconfigurable logic and/or hardware programmed that to operate as hardware emulator. Analog circuits in their basic form use only analog circuits and some examples can be a component comprising at least one; amplifier, oscillator, mixer, and/or filter. Mixed signal in their basic form only use both digital and analog circuits and some examples can be a component comprising at least one: Digital to Analog Converter (DAC), Analog to Digital Converter (ADC), Programmable Gain Amplifier (PGA), Power Supply control, Phase Lock Loop (PLL), and/or transistor behavior control over Process, Voltage and Temperature (PVT). The combination of digital logic components with analog circuit components can appear to behave like mixed signal circuits; furthermore, the examples that have been provided are not exhaustive as one knowledgeable in the arts understands. The PLL use a frequency reference that is typically derived from a crystal oscillator.
One of the critical design parameters of an electrical system is the generation of a stable and reliable oscillator. Quartz crystals oscillate under the influence of an electric field or can generate electric fields if exposed to stress. The quartz crystal, also called a crystal for short, can shaped into various sizes and thicknesses to achieve a myriad of resonant frequency behavior up to a fundamental frequency of about 30 MHz. As the crystal becomes thinner, the fundamental frequency goes higher. Higher order overtones above 30 MHz are also possible. Since the mechanical structure of the crystal oscillates, an electrical model of a tank circuit for the physical system can be determined. The electrical model is illustrated in FIG. 1. The mechanical mass of the crystal can be modeled by an inductance Lc, the stiffness of the crystal by a capacitance Cc and the heat loss by a resistance Rc. The shunt capacitance CSH is the capacitance that the crystal presents when it is not oscillating. Quartz crystals have superb frequency characteristics that allow these devices to be used in systems to track time. They provide a very stable clock for use in an integrated circuit.
One of the critical parameters in a crystal oscillator is the Equivalent Series Resistance (ESR). When the crystal oscillates at the fundamental frequency, the reactance of the inductance LC equals the absolute value of the reactance of the capacitance CC. When these two reactances are added together, they leave a net sum of zero. However, at this frequency, RC equals to ESR. The value of ESR can vary over a magnitude of order from crystals which operate at the same frequency. In a wireless system, the ESR of the crystal can range from 10 ohms to 150 ohms. Typically, a lower value of ESR implies that the crystal will have a higher cost because of the lower loss. The variations of this large range of ESR presents a problem in the design of the startup and biasing circuit to operate the crystal. If the value is too large, the crystal may not oscillate. If the value is too small, the circuit may exceed the drive level of the crystal and damage the crystal.
This drive level is an important criterion that the circuit must meet but not exceed in order to insure that the crystal is not damaged. The excessive drive level can cause the crystal to shift in frequency behavior, the device to age faster than expected, or worst yet, the device to over stress which results in the physical failure of the crystal. If the drive level is too small, the crystal may not oscillate at all. Currently, the approach to address these problems is to 1) design a family of circuits each designed to drive one of the limited range of drive levels forcing the chipmaker to offer several versions of their product which effectively increases their costs; or 2) design only one circuit and force the customer to buy a particular crystal with the specified drive range with the potential prospect of losing customers. An innovative technique will be described which overcomes both of these problems.
A crystal oscillator using Pierce configuration is a very common circuit design. The Pierce circuit only requires one inverter gain stage. The frequency spectrum at the output of an oscillator ideally should produce an oscillation only at the fundamental frequency. However, due to noise, the waveform at the output of the oscillator exhibits a frequency offset or spread of frequencies surrounding the fundamental frequency. One of the components of this noise causing the spread is called “l/f” noise or flicker noise. In any design of oscillators, the minimization of l/f noise is required to improve the system specifications. In the design of the crystal oscillator, another innovative technique is used to reduce the l/f noise.